In an internal combustion engine, as a rule, a catalyst for exhaust gas purification that is provided in an exhaust passage thereof purifies exhaust gas components. The purification of exhaust gas components by this catalyst for exhaust gas purification is efficiently carried out when the air-fuel ratio of a mixture burned in the internal combustion engine is within a predetermined range. Thus, air-fuel ratio control is performed by providing in the exhaust passage a sensor for outputting a signal corresponding to the concentration of oxygen in exhaust gas, detecting an actual air-fuel ratio of the mixture based on the output signal of this sensor, and feedback-controlling the amount of fuel injection such that the detected actual air-fuel ratio becomes equal to a target air-fuel ratio.
On the other hand, Japanese Patent Application Publication No. 2001-263015 (JP-A-2001-263015) discloses an apparatus that includes a lift amount change mechanism provided in an internal combustion engine to change the lift amount (in more detail, the maximum lift amount) of intake valves. In this apparatus, the action of the lift amount change mechanism is controlled such that a desired lift amount coincides with an actual lift amount. Thus, the intake valves are opened/closed with a lift amount suited for an engine operational state at each moment.
The lift amount of the intake valves has an individual difference resulting from the influence of a mounting error or the like. Therefore, the passage area of a communication region between an intake passage and a combustion chamber in the internal combustion engine is slightly different from a reference area thereof. Further, deposits may adhere to the intake valves as the engine is operated. In this case, the passage area changes to become different from the reference area thereof. This difference between the passage area and the reference area thereof constitutes a factor in a reduction of the accuracy in adjusting the amount of intake air and hence a reduction of the accuracy in adjusting the air-fuel ratio of the mixture. In an internal combustion engine in which the aforementioned feedback control is performed, the feedback control basically compensates for a change in the air-fuel ratio resulting from the difference between the passage area and the reference area thereof, thereby avoiding a reduction of the accuracy in adjusting the air-fuel ratio.
However, in the case where the passage area is different from the reference area thereof in the internal combustion engine provided, with the lift amount change mechanism, the change in the air-fuel ratio resulting from the difference differs depending on the mode of action of the lift amount change mechanism. Therefore, in the case where the mode of action of the lift amount change mechanism is frequently changed to suit an engine operational state at each moment, the change in the air-fuel ratio also changes frequently as the mode of action of the lift amount change mechanism is changed. Accordingly, when feedback control is performed simply based on a signal of a sensor as in the case of the apparatus disclosed in Japanese Patent Application Publication No. 2001-263015 (JP-A-2001-263015), it may become impossible to follow such frequent changes in the mode of action and adequately compensate for changes in the air-fuel ratio resulting from the changes in the mode of action.